The invention relates to a method and system for optimizing the performance of a rotodynamic multi-phase flow booster, such as a rotary pump or compressor for pumping a multi-phase gas/liquid mixture through a fluid transportation conduit.
Various types of rotodynamic pumps or compressors are known for boosting multi-phase flow. International patent application WO93/04288 discloses a contra-rotating pump or compressor of which the impeller blades are mounted on contra-rotating sleeves.
UK patent 2215408 discloses a system for controlling the gas-liquid ratio in a pump, wherein the gas-liquid ratio is maintained at a substantially constant level by extracting liquid from the downstream end of the pump and feeding the extracted liquid back to the pump inlet if the measured gas/liquid ratio exceeds a predetermined level.
U.S. Pat. No. 5,472,319 discloses an eccentric twin screw pump with liquid bypass controlled by a flexible diaphragm.
A disadvantage of the known multi-phase boosters is that the pumping efficiency varies in response to the density of the fluid mixture passing through the booster.
This variation can be particularly high in case gas and liquid slugs are passing alternatingly through the booster.
U.S. Pat. No. 3,568,771 discloses an electrical submersible oil well pump for lifting foamy crudes in which the rotary speed of the pump rotor(s) is varied as a function of the bulk density of the crude in the well bore. European patent application No. 0549439 discloses a multi-phase pump of which the speed of rotation is varied in response to variation of the gas/liquid ratio of the pumped multi-phase mixture.
An object of the present invention is to provide a method and system for further optimizing the performance of a rotodynamic flow booster, such as a gas/liquid compressor, such that the pumping efficiency is maintained at its best efficiency point if the density of the fluid mixture passing through the pump varies.
The method according to the invention for optimizing the performance of a rotodynamic multi-phase flow booster comprises measuring the density of the multi-phase fluid flow; and controlling the speed of rotation of a rotor of the pressure booster, characterized in that the density of the multi-phase fluid flow is measured at a location upstream of the rotary pressure booster, and in that the non-dimensional rotational speed xcexa9 of said rotor is controlled in relation to the measured non-dimensional density xcfx81 of the multi-phase fluid on the basis of the algorithm:
xcexa9=f.xcfx81n
wherein:
n is an exponent between xe2x88x921/3 and xe2x88x921 which is selected in relation to the mode of operation; and
f is a parameter which is determined by the geometry of the booster and the mode of operation, wherein the mode of operation is selected such that if the fluid mixture density xcfx81 varies either a) the mixture mass flow passing through the booster is maintained at a substantially constant level; or b) the power consumption of the booster is maintained at a substantially constant level; or c) the pressure difference between the outlet and the inlet of the booster is maintained at a substantially constant level.
Under the foregoing conditions the booster is expected to continue to operate at its best efficiency point (BEP) as with varying fluid mixture density and compositions the impellers or screws of the rotor(s) of the booster continuously create favourable fluid flow conditions such as a dispersed mist flow in which liquid droplets are finely dispersed in the gas phase or a dispersed bubble flow in which gas bubbles are finely dispersed in the liquid phase.
Suitably, the density and/or gas/liquid mass fraction of the multi-phase fluid mixture is measured by a gradio-venturi flowmeter or a wet gas tracer apparatus which is located in the conduit upstream of the flow booster.
The system according to the invention for optimizing the performance of a rotodynamic multi-phase flow booster comprises:
a density measuring device for measuring the density of the multi-phase flow; and
a rotary speed control unit for controlling the speed of rotation of a rotor of the booster in accordance with a predetermined algorithm, which controls the dimensionless speed of rotation xcexa9 of said rotor, characterized in that the density measuring device is located upstream of the flow booster, and in that the dimensionless speed of rotation xcexa9 is controlled in relation to the measured non-dimensional density xcfx81 of the multi-phase fluid on the basis of the algorithm:
xcexa9=f.xcfx81n
wherein:
n is an exponent between xe2x88x921/3 and xe2x88x921 which is selected in relation to the mode of operation; and
f is a parameter which is determined by the geometry of the booster and the mode of operation, wherein the mode of operation is selected such that if the fluid mixture density xcfx81 varies either a) the mixture mass flow passing through the booster is maintained at a substantially constant level; or b) the power consumption of the booster is maintained at a substantially constant level;
or c) the pressure difference between the outlet and the inlet of the booster is maintained at a substantially constant level.
The invention also relates to a rotodynamic multi-phase flow booster for use as a wet gas compressor and which is equipped with one or more rotor and/or stator parts having rotor and/or stator blades which are designed to transform liquid droplets which in use impinge upon the leading edges of the blades into a mist of smaller droplets which are and remain a finely dispersed mist in the gaseous phase.
Furthermore the invention relates to a rotor and/or stator blade suitable for use in such a wet compressor.
In such case the rotor blade comprises non-wettable sides, which are suitably coated with a polytetrafluorethylene or PTFE (sold by E. I. du Pont de Nemours and Company under the trademark xe2x80x9cTEFLONxe2x80x9d) and a leading edge having a larger roughness than the sides.
Suitably the leading edge of the blade(s) is covered by a random array of grains having a grainsize of at least about 50 microns, which grains are separated by distances which are on average less than twice the grainsize.
Alternatively the leading edge of the blade(s) is provided with a series of riblets which have a square or triangular cross-sectional shape, which are substantially aligned with the direction of flow of the multi-phase fluid mixture and which have mutual spacings of at least 50 microns.
In a wet gas compressor which has a rotor and/or stator which is equipped with such rotor and/or stator blades a favourable mist flow of small liquid droplets which are finely dispersed in the gaseous phase will be maintained also if the wet gas has a high liquid content which also contributes to operation of the wet gas compressor at its best efficiency point (BEP) in circumstances that the gas has a high liquid content, which may exceed 10% of the volume of the wet gas mixture.
The invention will be described in more detail with reference to the accompanying drawing, of which should not be construed to limit the scope of the invention.